Direct electrochemical detection of aliphatic compounds with electroactive functional groups based on electrocatalytic reactions at noble metal electrodes, chiefly Au and Pt, is well known. Electrochemical detection is a widely accepted means of detection in liquid and ion chromatography. Electrochemical detectors operate by applying an electrical potential to the working electrode in a flow-through cell. Such detectors typically employ a cell of three electrodes, which are a working electrode, a reference electrode and a counter electrode. Typically, the methodology uses multi-step potential waveforms which incorporate a detection operation along with the anodic and cathodic cleaning of the electrode surface. In a typical potential waveform, anodic detection occurs at a first potential with current sampling at the end of the time period of applying the potential. The potential then is stepped to a second higher potential for oxidative cleaning of the electrode surface, and subsequently to a third potential lower than the first or second potentials for cleaning by cathodic dissolution of the surface oxide formed at the first and/or second potentials. An analytical application of this method using amperometric detection, now known as Pulsed Amperometric Detection (PAD), has been demonstrated for alcohols, polyalcohols and carbohydrates (reducing and non-reducing); amines and amino acids (primary and secondary); aminoglycosides; and numerous sulfur compounds (except sulfate, sulfonic acids and sulfones).
Pulsed Coulometric Detection (PCD) has been described. The significant difference between PAD and PCD lies in the instrumental protocol related to measurement of the detection signal. The term “pulsed electrochemical detection” or “PED” is a general term that encompasses PAD and PCD. In PAD, electrode current is sampled or averaged over a time period (e.g., 16.7 ms) (at 1/60 Hz−1) whereas in PCD the response is electronically integrated over a single period or over an integral number of sequential periods. PCD inherently has a larger signal-to-noise ratio (S/N) because of the larger signal strength and because the integral of a 60-Hz correlated noise signal, a predominant form of noise in electronic instrumentation, remains at zero over the integration period.
Photometric detection suffers because of an inherently low sensitivity for aliphatic compounds without extensive π-bonding and because of baseline drift which accompanies a change in the refractive index of the mobile phase. Refractive index detection is strongly affected by concentration gradients and the baseline shift observed for even small changes in mobile phase composition can overwhelm the analyte signal.
The methods of PAD and PCD were introduced for detection of numerous aliphatic organic compounds with electroactive functional groups in conjunction with liquid chromatography (LC). Numerous PAD/PCD methods have become part of daily routine in academic and industrial laboratories. The majority of such methods utilize gold as a material for the working electrodes. The usage of platinum working electrodes has fallen behind the utilization of gold electrodes. Many analysts have found existing platinum-electrode based methods more difficult to use and less reproducible than those using gold electrodes.
Many analytes are detectable with working electrodes made of different materials. Various electrode materials exhibit different performance in response to sample matrix accompanying the analytes of interest. Example 1: Alcohols are detectable with platinum and gold electrodes. If alcohols are present in samples with high concentrations of sugars, platinum electrodes are preferable because they are less affected by excessive levels of sugars than gold electrodes. Example 2: Cyanide anion can be detected either with a silver electrode or with a platinum electrode. In samples with a high concentration of sulfides, silver electrodes are easily affected and have been observed to lose any response to cyanide after very few injections of high sulfide samples. Platinum electrodes are much more robust in comparison and keep providing detection response to cyanide even after many injections of high sulfide samples. Consequently, there is a need to provide more reproducible electrochemical platinum-based methodology suitable for an expanded list of analytes and which can be performed with high S/N.